In order to achieve optical processing of light from a light engine and to maintain the mechanical strength of an illumination apparatus with the light engine accommodated, a lampshade made of PC (polycarbonate) material is usually provided on the illumination apparatus. This lampshade is designed as a lamp tube in e.g. a tubular illumination apparatus, and a light engine, other electronic devices, and a heat sink can all be accommodated in the lamp tube. In a general tubular illumination apparatus, a heat sink of aluminum carrying a light engine is used, and this heat sink and the light engine are provided in a lamp tube made of PC, wherein the heat sink of aluminum can both achieve heat dissipation of the light engine and function as a frame, so as to reinforce the mechanical strength of the illumination apparatus. In addition, in another kind of illumination apparatus, a full PC lamp tube is used not only for achieving optical result such as light scattering, but also for reinforcing the mechanical strength of the illumination apparatus. The lamp tube made of PC material can realize the effect of light scattering, that is to say, viewed from outside of the illumination apparatus, this lamp tube can make light from the light engine softer and more uniform after light exits and make the illuminating result more suitable for human eyes. However, the lamp tube made of PC material might undergo deformation due to thermal expansion effect under a high temperature effect during the operation of the illumination apparatus, that is to say, both the size or the volume of the lamp tube might change, and even the PC lamp tube might bend over a long time period, which greatly adversely affects the normal function as both an optical element and a mechanical fastening element.